GETTYSBURG, Pa. — As gunshots ravaged the bodies of tens of thousands of soldiers at the Battle of Gettysburg, military doctors responded with a method of treatment that is still the foundation of combat medicine today.

Union Army Maj. Jonathan Letterman is remembered as the father of battlefield medicine for his Civil War innovations. He realized that organizing the medical corps was a key for any battle.

“For military medicine, in particular, the lessons that Dr. Letterman gave us are as true today as they were then,” said retired Lt. Gen. Ronald Ray Blanck, the former surgeon general for the U.S. Army.

Before the war, medical supplies were handled by regular quartermaster wagons, Blanck said, meaning they had to compete with “beans and bullets.”

The situation was so bad that, in some early Civil War battles, the wounded were left on the field for days, subject to the mercy of untrained troops and civilians.

In 1862, Letterman began to create an ambulance corps and three tiers of field hospitals: at the battlefield for simple wound dressing, nearby for emergency surgery, and behind the battle lines for long-term care and recovery.

Dale Smith, a professor of military history at the Uniformed Services University in Bethesda, Md., said Letterman’s innovations were so successful that Prussian and English observers wrote home to praise the system.

“There’s never been any question that he changed military medicine internationally,” Smith said.

But the Battle of Gettysburg was 150 years ago, and some have wondered how that could possibly be relevant for doctors in Iraq and Afghanistan, said George Wunderlich, director of the National Museum of Civil War Medicine in Frederick, Md.

Wunderlich recalled that about 10 years ago, a military member remarked that it was a shame the Civil War “has nothing to do with what we do today” with battlefield medicine.

But after Wunderlich told him how Civil War doctors resolved problems with transportation, training and even corruption, the man asked Wunderlich whether those topics could be turned into a one-day course.

Another man who complained that the Civil War training sessions were “unrealistic” called Wunderlich later after responding to Hurricane Katrina, where moving supplies was slow and difficult and even some cell towers were down.

“He says, ‘I’m so sorry. I’m in 1862 down here and now I get it,’ ” Wunderlich said.

Now, more than 5,500 military members and emergency responders have attended history courses run by the Museum of Civil War Medicine. The classes are designed to get people to think about how decisions get made in combat or crisis, and some are taught on battlefields at Gettysburg and Antietam.

The courses include topics such as courage and innovation; artillery and its effects; evacuation; and stress and fatigue.

“Our job is to use history to save the lives of people” today, Wunderlich said.